A Hydrophone System for Very Low Frequencies

Abstract

The Acoustic System Mark 1 was developed by the Naval Ordnance Laboratory for measuring and recording pressure variations in the frequency band between 0 and 200 c.p.s. It has had numerous applications including a seismic investigation of Bikini Atoll as a part of Operation Crossroads. A 1000-c.p.s. carrier is modulated by pressure variations applied to the hydrophone which comprises a variable inductance in one arm of a bridge. The diaphragm is resonant at about 1800 c.p.s. and can be used at depths up to 200 feet without pressure compensation. Although the hydrophone unit contains no amplifiers it may be used at distances exceeding 3 miles from the amplifying and recording system. Amplification of a million between motion of the diaphragm and motion of the recording pen are easily obtained. At a lower amplification the instrument may be used to follow changes in pressure caused by tides. Problems encountered in the development of the system included the mechanical design of an extremely rugged hydrophone and the electrical design of an oscillator of unusual amplitude stability and a circuit permitting easy and rapid balance of the bridge.